The light-emitting device or the white light-emitting device including the semiconductor light-emitting element and the wavelength conversion unit has attracted attention as a light-emitting device or a white light-emitting device of next generation expected to realize low power consumption, downsizing, high luminance, and wide range of color reproducibility, for which research and development have been conducted vigorously.
A GaN-based light-emitting element, a ZnO-based light-emitting element, or the like is used as the semiconductor light-emitting element. Light having a wavelength in the range from the longer-wavelength side of ultraviolet to blue (i.e., about 380 nm to about 480 nm) is used as the primary light emitted from the light-emitting element. The wavelength conversion units have been proposed, which use various kinds of phosphors suitable for converting the primary light to the secondary light.
An example for the light-emitting device having such a wavelength conversion function is disclosed in Japanese Patent Laying-Open No. 2004-071357. This document describes use of InN-based nano-crystals as phosphor but gives no detailed explanation for light emission characteristics thereof. Japanese Patent Laying-Open No. 2004-179644 discloses a light-emitting device taking account of light diffusion efficiency in each of plural phosphor layers that include light diffusing agents added in resin. In this document as well, there is no detailed explanation for light emission characteristics of the phosphor layers.
Meanwhile, in the field of application of the white light-emitting device, its use as a white lighting device for a backlight in a liquid crystal display (LCD) (especially in a liquid crystal television (TV) set) or for a flashlight for a camera is most important. The white lighting device is required to have high luminous efficiency and, at the same time, good color reproducibility. Particularly, it is preferable that the white lighting device is able to give white light analogous to blackbody radiation that is colorless.
Japanese Patent Laying-Open Nos. 2004-071357 and 2004-179644 give no detailed explanation for light emission characteristics regarding plural kinds of phosphors included in the light-emitting device and no technical consideration for effect of relative arrangement order of the plural kinds of phosphors.
From the standpoint of preventing the resin layers containing the phosphors from being deteriorated by ultraviolet light during the operating period of the light-emitting device, in the case of using a light-emitting element having an emission peak wavelength in the range of 400 nm to 500 nm corresponding to the wavelength range of blue, the excitation characteristics in the visible light range in the wavelength conversion unit (i.e., in the plural kinds of phosphors contained therein) suitable for the wavelength of the primary light have the critical meaning.
In other words, in order to exert the characteristics of the wavelength conversion unit containing plural kinds of phosphors to the maximum extent in a light-emitting device, it is important to configure an optimal stacked-layer state of the phosphors taking into consideration the excitation characteristics of the respective phosphors.
On the other hand, in order to enhance color reproducibility of the white light-emitting device, it is desirable to use a light source exhibiting high spectral purity for each of three primary colors of red, green and blue. It is conceivable to use an LED (light-emitting diode) or a semiconductor laser, for example, for each color light source. However, it is not possible to obtain a semiconductor light source having good luminous efficiency of green. Further, the luminance of each of the semiconductor light sources needs to be controlled independently from each other, which requires a drive circuit of a large scale.
As a method of implementing a white light source using one kind of semiconductor light source, Japanese Patent Laying-Open No. 10-242513 discloses a method of using a combination of a blue LED and a phosphor that is excited by the blue light and emits yellow light. With this method, however, there is a problem that the color reproducibility is poor, since the spectral purity of green is low, in addition to insufficient red component.
Similarly, there have been proposed various methods in each of which an LED for emitting light having a wavelength in the range from ultraviolet light of relatively longer wavelength to blue (i.e., from 380 nm to 480 nm) is combined with a phosphor that emits light of blue, green or red in response to the primary light emitted from the LED. For example, Japanese Patent Laying-Open No. 2004-327492 discloses a method of using a phosphor only for green light for the purpose of enhancing luminous efficiency of green. The method disclosed in this document, however, still requires a drive circuit of a large scale, since it uses plural kinds of semiconductor light-emitting elements.
Japanese Patent Laying-Open No. 2002-171000 discloses a method of combining an LED for emitting ultraviolet light and plural kinds of phosphors that emit lights of blue, green and red in response to the primary light emitted from the LED. The document however gives no suggestion about an optimal combination of the LED and the plural kinds of phosphors, particularly about the combination for producing a light source able to be regarded as blackbody radiation and at the same time for achieving high luminous efficiency and good color reproducibility over a wide range. In particular, with the phosphors disclosed in Japanese Patent Laying-Open No. 2002-171000, luminous efficiency is insufficient especially in the phosphor for emitting red light.
At present, a white light-emitting device mainly includes a light-emitting element for emitting blue light in combination with a trivalent cerium-activated (Y,Gd)3(Al,Ga)5O12 phosphor or a divalent europium-activated (Sr,Ba,Ca)2SiO4 phosphor for being excited by the blue light and then emitting yellow light.
Such a white light-emitting device, however, is not suitable for use as a backlight of a large LCD (especially for a LCD-TV), since its color reproducibility (NTSC ratio) is a little under 50%. That is, although the white light-emitting device having the combination of a semiconductor light-emitting element and a phosphor is advantageous in that it does not include mercury and can achieve low power consumption, downsizing and high luminance as compared to the cold cathode fluorescent lamp mainly used at present, there is an urgent need for improvement of its color reproducibility (NTSC ratio).
Here, the NTSC ratio is normalization with respect to an area of a triangle formed by connecting the chromaticity coordinates (x, y) of red (0.670, 0.330), green (0.210, 0.710) and blue (0.140, 0.080) in the XYZ color system chromaticity diagram defined by the National Television System Committee (NTSC).
Japanese Patent Laying-Open No. 2003-121838 discloses a prior art that has focused on color reproducibility (NTSC ratio) in the LCD. This document describes that a backlight light source has a spectral peak in the range from 505 nm to 535 nm; that an activator in a phosphor for emitting green light used in the light source includes europium, tungsten, tin, antimony, or manganese; and that MgGa2O4:Mn and Zn2SiO4:Mn are used as the phosphors for emitting green light in the embodiment. In the case that the peak wavelength of the light-emitting element is in the range from 380 nm to 450 nm, however, it cannot be said that every phosphor containing europium, tungsten, tin, antimony, or manganese can suitably be used. More specifically, MgGa2O4:Mn and Zn2SiO4:Mn mentioned in the embodiment of Japanese Patent Laying-Open No. 2003-121838 each have their very low luminous efficiency with the excitation light in the range from 380 nm to 450 nm, and thus these phosphors are not suitable for use in the present invention.
Further, Japanese Patent Laying-Open No. 2004-287323 describes that not only an RGB (red, green, blue)-LED having LED chips for emitting red, green and blue lights, respectively, contained in a package, but also a tri-color type fluorescent tube, an ultraviolet LED+RGB phosphors, an organic EL light source, and the like can be used as a backlight. In this document, however, there is no specific description regarding the RGB phosphors in the ultraviolet LED+RGB phosphors.